1. Field of the Invention
This invention relation to an improvement in frequency synthesizers in a phase locked loop circuit configuration for producing an output signal which differs from an input signal by a certain frequency difference, particularly adapted for use as a frequency signal generator in radio tranceivers, optical range measuring equipments and so on.
2. Description of the Prior Art
Conventionally, frequency synthesizers, such as a circuit made up of two quadrature phase shifters, two balanced modulators and one adder, have been used for local oscillators in various kinds of equipments. This kind of frequency synthesizer, however, has several disadvantages; that is, difficulty in precisely tuning the above mentioned circuit units, a large number of circuit elements, and difficulty in reducing, through a simple circuit, spurious components which are introduced into a local oscillator output signal due to a change in amplitude of the quadrature phase shifter's output as the input signal frequency varies (for instance, by channel switching in a tranceiver). To overcome the above disadvantages, recently a frequency synthesizer in phase locked loop (PLL) circuit configuration has been proposed as a frequency signal generator for producing a signal of frequency f.sub.R equal to a frequency f.sub.T -f.sub.IF or f.sub.T +f.sub.IF, where f.sub.T is the frequency of an input signal and f.sub.IF is an intermediate frequency which is predetermined as a frequency difference. Such PLL frequency synthesizer still has a specific disadvantage. In this synthesizer, there is a problem in that when frequency f.sub.T of the input signal changes by more than 2f.sub.IF, the PLL is thrown out of the capture range and then can no longer acquire lock.